Lasers without Inversion in Microcavities

The lasing characteristics of microcavity lasers with high coupling efficiency of spontaneous emission into the lasing mode are studied. Output power, population inversion, spectral linewidth and intensity noise are calculated as functions of pumping rate. When the spontaneous emission coupling efficiency into the lasing mode is close to one, a lasing threshold characterized by a sharp increase in an output power, a clamping of population inversion, a decrease in spectral linewidth and a peak in intensity noise, are observed at a pump rate in which a population inversion is not created yet. A physical interpretation for lasing without inversion in a microcavity is given on the basis of a photon recycling picture consisting of dissipation-free stimulated emission and re-absorption.